1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (AM LCD) device, and more particularly, to a method of displaying a color image using a field sequential liquid crystal display. Although the present invention is suitable in many applications, it is particularly useful for improving field sequential liquid crystal displays so as to increase a range of luminance and to decrease power consumption.
2. Discussion of the Related Art
Until recently, a cathode-ray tube (CRT) has usually been used for displays. However, flat panel displays are becoming more common because of their small depth, low weight, and low power consumption. Thin film transistor-liquid crystal displays (TFT-LCDs) are currently undergoing development to improve their resolution and to reduce their depth.
Generally, a liquid crystal display (LCD) device includes an upper substrate, a lower substrate, and an interposed liquid crystal layer. The upper and lower substrates have opposing electrodes such that an electric field applied across those electrodes causes the molecules of the liquid crystal to align according to the electric field. By controlling the electric field, a liquid crystal display device can produce an image.
The active-matrix liquid crystal display (AM LCD) device is probably the most popular type of LCDs because an AM LCD has high resolution and superior moving image properties. A typical AM LCD has a plurality of switching elements and pixel electrodes that are arranged in a matrix on the lower substrate. Therefore, the lower substrate of an active-matrix liquid crystal display is often referred as an array substrate.
The structure of a conventional active-matrix liquid crystal display is described with reference to FIG. 1, which illustrates a cross-section of a pixel region. The active-matrix liquid crystal display 2 includes a liquid crystal panel 10 and a back light 50. The liquid crystal panel 10 includes a color filter substrate 20 and an array substrate 40 that face each other across a liquid crystal layer 30. On the color filter substrate 20 is a color filter layer 22 that includes a black matrix 22a (for preventing light leakage) and color sub-filters 22b, including red (R), green (G), and blue (B) sub-filters. The color filter substrate 20 also includes a common electrode 24, which is one of the electrodes used in applying a voltage across the liquid crystal layer 30.
Still referring to FIG. 1, a thin film transistor, which functions as a switching element, and a pixel region are formed on the array substrate 40. The thin film transistor and the pixel region are disposed across from the color filter substrate 20. A pixel electrode 42 electrically connects to the thin film transistor (which is formed in the region T). The pixel electrode 42 functions as the other electrode used for applying voltage across the liquid crystal layer 30. The pixel electrode 42 is located in the pixel region “P”. A back light 50 is disposed under the array substrate 40. The back light radiates light onto the liquid crystal panel 10. The back light 50 includes a light source 52 and a plurality of panels 54 that uniformly radiating light from the light source 52 onto the liquid crystal panel 10.
The liquid crystal display device 2 uses optical anisotropy and polarization properties of the liquid crystal molecules to produce a desired image. That is, by applying a voltage across the liquid crystal molecules (which have a long, thin structure and which have a pretilt angle) the alignment of the liquid crystal molecules changes. Thereafter, light from the back light 50 is polarized by the optical anisotropy of the liquid crystal. That polarized light is then controllably passed through the color filter layer to produce a color image.
Refer now to FIG. 2 for another view of a liquid crystal display. As shown, the liquid crystal panel 10 includes the array substrate 40, the color filter substrate 20, and the interposed liquid crystal layer 30. A plurality of gate bus lines 46 are horizontally arranged, and a plurality of data bus lines 48 are vertically arranged on the array substrate 40. Those bus lines define a plurality of pixels (between the bus lines). Thin film transistors “T” are formed near the intersections of the gate bus lines 46 and the data bus lines 48. A pixel electrode 42 within each pixel region is connected to an associated thin film transistor “T”. The common electrode 24 and the color filter layer 22 (with the color sub-filters) are formed on the color filter substrate 20.
In the conventional liquid crystal display device described above, the process for displaying a color image is as follow. First, liquid crystal alignment is changed by applying a voltage across each pixel of the liquid crystal layer. The incident light from the back light is polarized by irradiating it through the liquid crystal having the aligned liquid crystal. Then, a color image pixel is produced by passing the polarized light through the color sub-filters red (R), green (G), and blue (B). Therefore, in the conventional liquid crystal display device it is necessary to include red (R), green (G), and blue (B) color sub-filters to produce a color image.
The color filter layer is typically manufactured using either a dye-method (in which a dye resin is formed on a transparent substrate) or a pigment-spraying method (in which a pigment is sprayed on a transparent substrate). However, those methods have problems. First, the materials used are expensive, and the methods tend to consume a lot of those materials. The results is a relatively high manufacturing cost. Second, the materials that are used have a maximum light transmissivity of about 33%, necessitating a bright back light to effectively display a color image. Such a bright back light results in relatively high power consumption. Furthermore, if the color filter layer is thick, the color properties are improved, but the light transmissivity is reduced. On the other hand, if the color filter is thin, the light transmissivity is improved, but the color properties are poor. Therefore, a manufacturing process having great precision is required. However, since such is not available, the result is a low production yield and an inferior product.
Many studies and experiments have been performed to enable a full color display that does not require a color filter. While such studies and experiments had not proven commercially successful, the development of new liquid crystal modes, such as Ferroelectric Liquid Crystal (FLC), Optical Compensated Birefringent (OCB), field sequential, and Twisted Nematic (TN) displays open new possibilities in producing full color displays.
The structure of the field sequential liquid crystal display device is explained with reference to FIG. 3, which illustrates a part of a field sequential liquid crystal display device. As shown, a field sequential liquid crystal display device includes a common electrode substrate 65 and an array substrate 80 that are spaced apart in a facing relationship. A liquid crystal layer 70 is disposed between the common electrode substrate 65 and the array substrate 80. A plurality of gate bus lines 82 is horizontally arranged, while a plurality of data bus lines 84 is vertically arranged on the array substrate 80. Those bus lines define a plurality of pixels. Thin film transistors are formed at the intersections of the gate bus lines 82 and the data bus lines 84. Furthermore, a pixel electrode 86 that is connected to a thin film transistor is in each pixel region.
As shown in the circle of FIG. 3, each thin film transistor “T” is a switching element having a gate electrode “G”, a source electrode “S” and a drain electrode “D”. The gate electrode “G” is connected to a gate line 82, the source electrode “S” is connected to a data line 84, and the drain electrode “D” is connected to a pixel electrode 86. A common electrode 66 is formed on the common electrode substrate 65. However, unlike in the LCD shown in FIGS. 1 and 2, the common electrode substrate 65 does not have a color filter. Still referring to FIG. 3, a back light 90 is disposed under the liquid crystal panel 60. That back light radiates light onto the liquid crystal panel 60. The back light 90 of the field sequential liquid crystal display device has three different light sources, which can produce three different colors of light, red (R) 94a, green (G) 94b, and blue (B) 94c. Additionally, a plurality of panels 92 ensures uniform dispersion of light from the back light (R, G, and B) onto the liquid crystal panel. The field sequential liquid crystal display device further includes an external driving circuit for applying signals to produce a desired image. The external driving circuit includes a gate scan input driver 98 that apples electric pulses to the horizontal gate bus lines 82 and a data input driver 96 for applying image signals to vertical data bus lines 84.
The back light 90 can be two different kinds. One, as shown in FIG. 4a, is a wave guide mode back light in which Red, Green and Blue light sources are disposed in a lower corner of the array substrate 80. The other, as shown in FIG. 4b, has Red, Green and Blue light sources disposed directly under the array substrate 80 in a repeated ordering of Red, Green and Blue.
A color image display and driving method for a field sequential liquid crystal display device will be explained with reference to FIGS. 3 and 5. FIG. 5 illustrates a flow chart of a method of producing a color image using a conventional field sequential liquid crystal display device. Initially, frame-based image signals are input from a data input driver onto the data bus lines. Each frame-based image signal is comprised of first, second and third sub-frame image signals that are related to Red, Green and Blue color images that are to be produced in respective sub-frames. Those sub-frame image signals selectively turn on the thin film transistors during a sub-frame so as to align the liquid crystal in each sub-frame periods. With the liquid crystal properly aligned the light source associated with that sub-frame (Red, Green, and Blue) is then turned on and off to produce an image. The overall perception of the three sub-frames produces a color frame.
Thus, in a field sequential liquid crystal display device the frame-based image signals include signals for three light colors (Red, Green and Blue), and each color image signal is applied during a sub-frame period. Further, the liquid crystal molecules are arranged during each sub-frame by selectively turning on the thin film transistors. By properly sequencing turning on and off the light sources with the sub-frame liquid crystal molecule alignment a color image is produced during each frame. Because the Red, Green and Blue images in each frame appear to be blended together, when observed a color image results.
The foregoing will be explained in more detail. Referring now to FIG. 5, Red image signals are applied to the data bus lines by the data input driver 96 during a first sub-frame period (which is one-third of a full frame period). At the same time the gate scan input driver 98 selectively applies gate pulse voltages to the gate line. Namely, as shown in FIG. 3, when a gate pulse voltage is applied to the gate line Gi, the thin film transistors connected to that gate line are turned on in accord with the intensity or the pulse width of the gate pulse voltage. Reference step 100 of FIG. 5. Because the turned-on thin film transistors connect to the data lines, the Red component image signals from the data input driver are applied across the liquid crystal cells associated with the turned-on thin film transistors. Charges accumulate across those liquid crystal cells, which then arrange the liquid crystal molecules, reference step 105 of FIG. 5. Then, a gate pulse voltage is applied to the gate line Gi+1, which causes the thin film transistors connected to the gate line Gi+1 to turn on, causing charges to accumulate across their liquid crystal cells. Furthermore, the thin film transistors connected to the gate line Gi are turned off and their accumulated charges are stored until the gate line Gi is driven during the next sub-frame. When all of the thin-film transistors have turned on, the liquid crystal molecules are properly aligned. Thereafter, the Red light source of the back light is turned on and off (in step 110) to produce a Red component of an image (in step 115). The first sub-frame is then complete.
Next, during the second sub-frame Green image signals are applied to the data bus lines by the data input driver 96. At the same time the gate scan input driver 98 selectively applies gate pulse voltages to a gate line. Namely, as shown in FIG. 3, when a gate pulse voltage is applied to the gate line Gi, the thin film transistors connected to that gate line are turned on in accord with the intensity or the pulse width of the gate pulse voltage. Reference step 120 of FIG. 5. Because the turned-on thin film transistors connect to the data lines, the Green component image signal voltages from the data input driver are applied across the liquid crystal cells associated with the turned-on thin film transistors. Charges then accumulate across those liquid crystal cells, which then arrange the liquid crystal molecules, reference step 125 of FIG. 5. After the Green image signals are all accumulated and the liquid crystal is properly aligned, the Green light source of the back light is turned on and off (in step 130). Thus a Green component of the image is produced during the second sub-frame (in step 135).
Finally, during the third sub-frame Blue image signals are applied to the data bus lines by the data input driver 96. At the same time the gate scan input driver 98 selectively applies gate pulse voltages to a gate line. Namely, as shown in FIG. 3, when a gate pulse voltage is applied to the gate line Gi, the thin film transistors connected to that gate line are turned on in accord with the intensity or the pulse width of the gate pulse voltage. Reference step 140 of FIG. 5. Because the turned-on thin film transistors connect to the data lines, the Blue component image signal voltages from the data input driver are applied across the liquid crystal cells associated with the turned-on thin film transistors. Charges then accumulate across those liquid crystal cells, which arrange the liquid crystal molecules, reference step 145 of FIG. 5. After the Blue image signals are all accumulated and the liquid crystal is properly aligned the Blue light source of the back light is turned on and off (in step 150), and thus a Blue component of the image is displayed during the third sub-frame (in step 155).
The period of one frame is typically one-sixtieth of a second. Thus, each sub-frame is one-third of one frame period, i.e., one-one hundred eightieth of a second. As explained previously the Red, Green and Blue image components are sequentially displayed so as to be perceived as a composite color image by an observer. As an example, if a white image is to be displayed, each of the Red, Green and Blue image components has the same luminance. Thus, a white image can be displayed by mixing image components having the same intensity together. The luminance of the displayed image of a field sequential liquid crystal display device depends on the luminance of the back light. That luminance in turn depends on the transmissivity of the elements constituting the liquid crystal panel and the transmissivity of the liquid crystal layer. That is, each light source passes through the liquid crystal panel and each is polarized by the liquid crystal layer. Thus, the luminance of each light source (Red, Green and Blue) is diminished by the transmissivity of the liquid crystal panel and the transmissivity of the liquid crystal layer (which is varied by the alignment of the liquid crystal molecules).
Because the transmissivity of the liquid crystal panel has a specific value determined by the elements constituting the liquid crystal panel, and because the back light has only two luminance values (corresponding to turned-on and turned-off), the luminance of an image displayed on the liquid crystal display screen is controlled by the transmissivity of the liquid crystal, which depends on the alignment of the liquid crystal molecules. Therefore, the luminance range of the conventional field sequential liquid crystal display device is relatively limited. Additionally, the overall power consumption when driving the back light is relatively high because each light source (Red, Green and Blue) is turned on and off to produce the same luminance.